1. Field of the Invention
The present invention relates to a data processing method for correcting heating data for a thermal head of a thermal printer, to eliminate influence of heat energy accumulated in the thermal head such that print quality may not be degraded by the heat accumulation. The present invention relates also to a data processing apparatus for this method.
2. Background Arts
There are thermosensitive recording type thermal printers and thermal transfer type thermal printers. The former heats a thermosensitive recording sheet directly with a thermal head, to cause the sheet to develop color. The latter heats the back of an ink ribbon placed upon a recording sheet to transfer ink to the recording sheet. The thermal printer has a thermal head which has an array of heating elements arranged on a ceramic substrate. The array of heating elements correspond to a line of pixels, and the heating elements are each individually driven to record a dot at a time, as the recording sheet is conveyed in a perpendicular direction to the heating element array. Thus an image is printed line by line on the recording sheet. Hereinafter, the direction along which the heating element array extends will be called a main scan direction, whereas the direction along which the recording sheet is conveyed will be called a sub scan direction.
In the thermosensitive recording type and sublimation ink transfer type thermal printing, one dot constitutes one pixel of the printed image, and has a variable density including a zero level, that is designated by image data for each pixel. However, where the heating elements are driven simply in accordance with the image data, the densities of the dots can deviate from the designated densities because of heat accumulation in the thermal head.
Most of heat energy generated from the heating elements is used for recording, but the rest stays unused or dissipates. The unused heat energy is mainly accumulated in a glazing layer which is formed between the heating elements and ceramic substrate. Part of the accumulated heat energy is transmitted from the glazing layer to the ceramic substrate and is accumulated therein, or partly transmitted further to an aluminum plate supporting the substrate and is accumulated therein. From the aluminum plate, the heat energy is partly transmitted to a radiation plate, and radiates from the radiation plate. Hereinafter, the layers disposed under the heating elements will be referred to as heat accumulating layers. The heat energy accumulated in this way is partly transmitted back to the heating elements and added to the heat energy presently generated from the heating elements, thereby rising the densities of the dots from the designated values. Where density should change steeply from high to low, the change becomes gentler in the printed image, so the contour or edge of the printed image becomes vague. Since the heat accumulation in the thermal head gradually increases as the recording proceeds, it results a gradual density change overall the printed image, called shading. That is, the density is generally low at the start of recording, and as the recording proceeds, it becomes higher as a whole.
In order to prevent the above mentioned degrading of the printed image caused by the heat accumulation, U.S. Pat. No. 5,800,075 suggests a data processing method, wherein the heat accumulation in the respective heat accumulating layers of the thermal head is estimated on the basis of input image data, and the input image data is corrected with correction data determined for each pixel based on the estimated heat accumulation.
According to this method, because the heat energy values transmitted to the heat accumulating layers are calculated by linear conversion of the generated heat energy values, if a high density area, e.g. black characters or lines, is included in a middle density area, peripheral portions around the high density area have higher densities than expected, and the image quality is degraded.
The thermal printers also have a problem that the environmental temperature has influence on the densities of the printed dots because the heat energy generated from the heating elements of the thermal head is transmitted to and diffused in the ink ribbon, the recording sheet, a platen roller, a platen drum or the like. That is, if the environmental temperature is pretty high or low, the dot densities get higher or lower than expected, even while the heat energy generated from the heating elements is proper. As a solution for this problem, it is known in the art to adjust voltage applied to the thermal head depending upon the temperature of the thermal head and the environmental temperature.
Since the above data processing method is on the presumption that the head drive voltage applied to the thermal head is kept unchanged, this method rather causes a problem where the head drive voltage is changed according to the temperature of the thermal head and the environmental temperature. That is, the dot densities would deviate from the designated values as the recording proceeds.
In view of the foregoing, an object of the present invention is to provide a data processing method for correcting heating data for a thermal head and an apparatus therefor, whereby the influence of heat accumulation in the thermal head on the recording density is well eliminated taking account of head drive voltage applied to the thermal head.
Another object of the present invention is to provide a data processing method for correcting heating data for a thermal head to eliminate influence of heat accumulation in the thermal head on recording density, and an apparatus therefor, whereby conditions of heat energy accumulated in the respective heat accumulating layers are estimated with high accuracy, taking account of heat transmission properties between the heat accumulating layers, and thus the influence of heat accumulation on the recording density is well eliminated.
On the assumption that a thermal head has an array of heating elements arranged in a line and first to Nth heat accumulating layers disposed under the heating elements in this order from the side of heating elements, and the heating elements are driven in accordance with corrected heating data, to print one line after another on a recording sheet, one pixel of each line corresponding to one heating element of the array in regular sequence, and that a drive voltage to be applied across the heating elements is determined according to a temperature of the Nth heat accumulating layer and an environmental temperature around the thermal head, a data processing method of the present invention comprising the steps of:
A. determining first to (Nxe2x88x921)th coefficients for the first to (Nxe2x88x921)th heat accumulating layers based on heat transmission properties between the first to Nth heat accumulating layers, and a Nth coefficient for the Nth heat accumulating layer based on the drive voltage for the thermal head and on the heat transmission properties between the first to Nth heat accumulating layers;
B. obtaining first to Nth correction data for each pixel of a subject line to print, by multiplying first to Nth heat accumulation data by the first to Nth coefficients respectively, the first to Nth heat accumulation data being representative of heat accumulation amounts in the first to Nth heat accumulation layers respectively, and previously stored in relation to each heating element of the array;
C. obtaining corrected heating data for each pixel of the subject line, from basic heating data representative of a heat energy value to be applied to the recording sheet for recording the pixel and the first to Nth correction data for the pixel;
D. obtaining a new series of first heat accumulation data by multiplying the corrected heating data of the subject line by a coefficient, multiplying the previously stored first heat accumulation data by a coefficient, and adding multiplication results in pixel-to-pixel correspondence;
E. obtaining a new series of Jth heat accumulation data, J being 2 to N, by multiplying the previously stored (Jxe2x88x921)th heat accumulation data by a coefficient, multiplying the previously stored Jth heat accumulation data by a coefficient, and adding multiplication results in pixel-to-pixel correspondence; and
F. storing the new series of first to Nth heat accumulation data in place of the previously stored first to Nth heat accumulation data, while the subject line is printed in accordance with the corrected heating data.
The above steps B to F are repeated for each line to print.
In this way, the drive voltage for the thermal head is taken into consideration on determining the Nth correction data, so the corrected heating data is effective for printing the data at the expected density even where the drive voltage is changed according to the head temperature and the environmental temperature.
A data processing apparatus of the present invention comprises:
a voltage determining means for determining a drive voltage to be applied to the thermal head, according to a temperature of the Nth heat accumulating layer and an environmental temperature around the thermal head;
a memory means for storing first to Nth heat accumulation data in relation to each heating element of the array, the first to Nth heat accumulation data being representative of heat accumulation amounts in the first to Nth heat accumulation layers respectively;
first to Nth multipliers multiplying the first to Nth heat accumulation data by first to Nth coefficients respectively, to obtain first to Nth correction data for each pixel of a subject line to print;
a coefficient determining means for determining the Nth coefficient based on heat transmission properties between the first to Nth heat accumulating layers, and on the drive voltage for the thermal head;
a correcting means for correcting basic heating data of the subject line, with the first to Nth correction data and a coefficient in pixel-to-pixel correspondence, to produce corrected heating data of the subject line, the basic heating data being representative of a heat energy value for each pixel to be applied to the recording sheet for recording the pixel;
a first calculator for obtaining a new series of first heat accumulation data by multiplying the corrected heating data of the subject line by a coefficient, multiplying the previously stored first heat accumulation data by a coefficient, and adding multiplication results in pixel-to-pixel correspondence; and
second to Nth calculators for obtaining a new series of Jth heat accumulation data, J being 2 to N, by multiplying the previously stored (Jxe2x88x921)th heat accumulation data by a coefficient, multiplying the previously stored Jth heat accumulation data by a coefficient, and adding multiplication results in pixel-to-pixel correspondence, wherein the new series of first to Nth heat accumulation data are written on the memory device in place of the previously stored first to Nth heat accumulation data, during the printing of the subject line, and are used for obtaining first to Nth correction data for a next line to print.
According to another aspect of the present invention, a data processing method for correcting heating data for a thermal head to eliminate influence of heat accumulation in the thermal head on recording density, comprises the steps of:
A. obtaining first to Nth correction data for each pixel of a subject line to print, by multiplying first to Nth heat accumulation data by first to Nth coefficients respectively, the first to Nth heat accumulation data being representative of heat accumulation values in the first to Nth heat accumulation layers respectively, and previously stored in relation to each heating element of the array;
B. obtaining corrected heating data for each pixel of the subject line, from basic heating data representative of a heat energy value to be applied to the recording sheet for recording the pixel and the first to Nth correction data for the pixel;
C. converting the corrected heating data for the subject line, into modified heating data through a non-linear function that is predetermined based on heat transmission properties between the heating elements and the first heat accumulating layer;
D. obtaining a new series of first heat accumulation data by multiplying the modified heating data of the subject line by a coefficient, multiplying the previously stored first heat accumulation data by a coefficient, and adding multiplication results in pixel-to-pixel correspondence;
E. obtaining a new series of Jth heat accumulation data, J being 2 to N, by multiplying the previously stored (Jxe2x88x921)th heat accumulation data by a coefficient, multiplying the previously stored Jth heat accumulation data by a coefficient, and adding multiplication results in pixel-to-pixel correspondence; and
F. storing the new series of first to Nth heat accumulation data in place of the previously stored first to Nth heat accumulation data, while the subject line is printed in accordance with the corrected heating data.
The above steps A to F are repeated for each line to print.
Since the heat accumulation data for the first heat accumulating layer is determined based on the modified heating data that is obtained by converting the corrected heating data for the subject line through a non-linear function that is predetermined based on heat transmission properties between the heating elements and the first heat accumulating layer, conditions of heat energy accumulated in the respective heat accumulating layers are estimated with high accuracy.
According to a preferred embodiment of the invention, the heat accumulating layers of the thermal head comprise a glazing layer, a ceramic substrate and an aluminum plate laid on one another in this order from the side of heating element, and the glazing layer is hypothetically divided into a number of heat accumulating layers arranged vertically from each other, to obtain the heat accumulation data and the correction data for each of the hypothetically divided heat accumulating layers. Thereby, thermal conductivity of the glazing layer is taken into consideration on estimating condition of heat energy accumulated in the glazing layer.
A data processing apparatus for correcting heating data for a thermal head to eliminate influence of heat accumulation in the thermal head on recording density, the thermal head having an array of heating elements arranged in a line and first to Nth heat accumulating layers disposed under the heating elements in this order from the side of heating elements, the heating elements being driven in accordance with corrected heating data, to print one line after another on a recording sheet, one pixel of each line corresponding to one heating element of the array in regular sequence, the data processing apparatus comprising:
a memory means for storing first to Nth heat accumulation data in relation to each heating element of the array, the first to Nth heat accumulation data being representative of heat accumulation values in the first to Nth heat accumulation layers respectively;
first to Nth multipliers for multiplying the first to Nth heat accumulation data by first to Nth coefficients respectively, to obtain first to Nth correction data for each pixel of a subject line to print;
a correcting means for correcting basic heating data of the subject line, with the first to Nth correction data and a coefficient in pixel-to-pixel correspondence, to produce corrected heating data of the subject line, the basic heating data being representative of a heat energy value for each pixel to be applied to the recording sheet for recording the pixel;
a conversion means for converting the corrected heating data for the subject line, into modified heating data through a non-linear function that is predetermined based on heat transmission properties between the heating elements and the first heat accumulating layer;
a first calculator for obtaining a new series of first heat accumulation data by multiplying the corrected heating data of the subject line by a coefficient, multiplying the previously stored first heat accumulation data by a coefficient, and adding multiplication results in pixel-to-pixel correspondence; and
second to Nth calculators for obtaining a new series of Jth heat accumulation data, J being 2 to N, by multiplying the previously stored (Jxe2x88x921)th heat accumulation data by a coefficient, multiplying the previously stored Jth heat accumulation data by a coefficient, and adding multiplication results in pixel-to-pixel correspondence, wherein the new series of first to Nth heat accumulation data are written on the memory device in place of the previously stored first to Nth heat accumulation data, during the printing of the subject line, and are used for obtaining first to Nth correction data for a next line to print.